Field
The present disclosure generally relates to techniques for reworking structures, and deals more particularly with a method of detecting leaks and reworking composite laminate structures.
Background
Localized areas of composite structures sometimes need to be reworked in order to eliminate non-conformities, including inconsistencies and undesired alterations. These non-conformities may be created either during manufacturing, or after the structure has been placed in service. For example, composite laminate skins used on aircraft airframes sometimes experience impact damage caused by an impact with a foreign object during flight, or contact with service vehicles and equipment while on the ground.
Currently, in order to eliminate a non-conformity in a composite laminate aircraft skin, material is removed from a local area of the skin which includes and surrounds the non-conformity. In many cases, the needed material removal does not extend through the full thickness of the skin. Uncured composite material, referred to as a rework patch, is then placed over the skin and a vacuum bag is installed over the rework patch. In some cases, a pre-cured patch is installed and adhesively bonded in place. Heat is applied to the rework area in order to thermally cure the rework patch, or the bonding adhesive, following which the vacuum bag is removed and the rework area is fared-in with the existing skin. Where a non-conformity is severe or extensive, as may occur during a substantial impact with the skin, a void such as a crack or fracture may be created that extends through from the frontside to the backside of the skin. This type of through-the-thickness void represents a leak path that may allow air to be drawn into the rework area from the backside of the skin when the bag is evacuated. As a result of such leaks, air may be allowed to enter rework patch, causing undesired voids or porosities in the patch.
If an area requiring rework is determined to have a leak, or a likelihood of a leak, a vacuum bag may be applied to the backside of the structure to prevent air from reaching the rework patch through the leak path. While this approach is satisfactory in some applications, it has several disadvantages. First, the installation of a second vacuum bag on the backside of the skin is time-consuming, costly and labor intensive. Second, it is possible that the dimensions of the backside bag may not be large enough to seal off all leaks, due to the fact that some leaks can be located relatively far from the frontside area being reworked. Third, depending upon the location of the area requiring rework, it may not be possible to install a backside bag because of structural obstructions such as, for example and without limitation, where a stringer underlies the backside of the skin. In some cases, the obstruction may be disassembled to provide enough clearance for installation of a backside bag, however the disassembly process is time-consuming and labor intensive.
Accordingly, there is a need for a method of reworking a structure, such as a composite laminate skin, which permits leak detection from the frontside of the skin and which does not require access to the backside of the skin.